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More than 12 million Americans travel on passenger-rail lines each weekday, and because of its open nature, rail transit is considered an attractive terrorist target. While there have been no successful attacks on U.S. rail systems recently, attacks on passenger-rail systems around the world — such as the London Underground in 2005 — highlight the vulnerability of rail travel and the importance of rail security for passengers.

The study by RAND, a nonprofit research organization, uses a generic intracity rail system with characteristics similar to existing American systems. An interdisciplinary team of RAND researchers identified 17 security improvement options — such as canine teams, vehicle surveillance systems, and blast resistant containers — and assessed their relative effectiveness when deployed in different parts of the rail system.

“Millions ride the nation's railways every day, and it is critical to protect them from terrorist attacks. But we need ways to do so while getting the most for the money we invest,” said Jeremy Wilson, the study's lead author. “By design, rail systems are open and accessible by large numbers of people, and for this reason are difficult to secure.”

The framework RAND researchers developed gives transit officials a guide to help them evaluate their systems and determine the best, and most efficient, ways to improve safety. The study is based on a composite system in order to avoid disclosing confidential details about any specific rail system in the United States.

Brian Jackson, co-author of the report, said terrorists have demonstrated the ability to change strategies and tactics in response to security measures. As a result, passenger-rail officials and policy makers need to adapt in order to protect their riders.

“Rather than providing a static defense, security planners should review their plans regularly to ensure that they remain relevant to any changes in the terrorists' targeting methods,” Jackson said.

The study focuses on addressing vulnerabilities and limiting consequences, the two components of risk rail security measures can most influence. Additionally, researchers focused on intracity heavy rail systems—characterized by high speed and rapid acceleration cars, such as the Metro in Washington, D.C., MARTA in Atlanta and the Red Line in Los Angeles—and did not include light rail or commuter rail, such as Amtrak.

The study finds that 80 percent of the worldwide attacks on rail systems were bombings, followed by sabotage (6 percent) and armed attack (6 percent). Explosives accounted for 77 percent of the weapons used in rail system terrorist incidents, with 8 percent of the incidents involving hoaxes or threats.

Using the generic rail system as the intended target, researchers took data on past terrorist attacks on rail systems from the RAND-Oklahoma City National Memorial Institute for the Prevention of Terrorism (MIPT) Terrorist Incident Database to develop a risk assessment.

Researchers examined 11 potential attack locations in a rail system, such as underground infrastructure, ground-level stations, and elevated rail lines, and subjected them to eight different forms of attack, including bombings, incendiaries, and unconventional weapons.

Among the lessons learned:

Security measures must address the threat of explosive devices, but not exclusively; while attacks such as those from chemical and radiological weapons are unlikely, their potentially serious results mean they merit attention.

Most terrorist attacks on rail systems have resulted in few or zero deaths; as a result, even if security measures prevent only the largest-scale attacks, they could significantly reduce the human costs associated with the threat.

Some rail attack modes are more of a concern than others. For example, the use of small explosives is a high or medium risk for most targets, while hoaxes or threats pose only a risk for a few targets.

Because terrorists who are trying to kill and injure people target densely populated areas, attacks inside train cars, targeting train cars from outside, and in densely populated stations are of major concern.

“While it isn't practical, or desirable, to base security planning on every conceivable terrorist threat, it's important that security personnel not limit their planning to the obvious attacks from the past,” he said. “We simply can't be certain what terrorists will do next.”

Researchers identified four broad categories of cost-effective security measures for system operators to consider in terms of effectiveness per dollar payoffs:

Relatively inexpensive solutions with the highest effectiveness, such as enhanced security training.

Additional inexpensive solutions to consider with reasonable levels of effectiveness; for example, installing retractable bollards at entrances and exits of the central system operations center and power plant.

Relatively more expensive solutions with highest effectiveness, such as installing fixed barriers at curbsides adjacent to all entrances and passageways leading to ground-level and underground stations.

Wilson cautions that the options that arise from an analysis using the framework are specific to the particular system analyzed and the analysis captures a point in time. As a result, the appeal of each option is influenced by cost and perceived effectiveness.

The research was sponsored by an award from the National Institute of Justice, the research, development and evaluation arm of the U.S. Department of Justice.

The research was conducted by RAND Infrastructure, Safety and Environment, which conducts research and analysis to improve the development, operation, use and protection of society's essential man-made and natural assets, and to enhance the safety and security of individuals in transit, at work and in their communities.

The RAND Corporation is a research organization that develops solutions to public policy challenges to help make communities throughout the world safer and more secure, healthier and more prosperous. RAND is nonprofit, nonpartisan, and committed to the public interest.